1. Field of the Invention
The present invention relates to a position detecting device which calculates values including coordinate values of a position specified using a position indicator.
2. Description of Prior Art
In Japanese Patent Application Laying- open No. H3(1991)-147012 (hereinafter referred to as "the prior art"), the applicant has proposed means to reduce detecting time, expanding selectivity for measuring conditional, and compacting its size which was provided within a position detecting device for transmitting and receiving electromagnetic waves between a sensing unit and a position indicator to calculate coordinate values of a specified position indicated by a position indicator.
Firstly, describing briefly the prior art, the sensing unit comprising a large number of loop coils disposed in parallel with respect to each other in the direction of position detection is arranged to divide the loop coils into a plurality of groups, and one coil of each group is selected respectively, and an alternating current is applied to all coils selected at a time to generate electromagnetic waves resonating a turning circuit inside the position indicator. The electromagnetic wave transmitted by the turning circuit is then received by the selected loop coils to generate an induced voltage. Such a procedure is sequentially performed to each of the selected loop coils of the respective group to detect the induced voltage generated in the respective coil of the group i.e. an amplitude and phase angle of a received signal are thus detected to calculate the coordinate values of the specified position based on them.
One of the features of the prior art is that the selecting order of the loop coils of the groups is predetermined such that the respective pattern of the signal amplitude obtained therefrom is corresponding to a specific one of the loop coil located at the position indicator in order to determine the position of the position indicator.
Further, another feature of the prior art is that it uses the alternating signal, which has a frequency allowed to be suitably set, as a detecting signal and that signal generation and received signal processing means are provided with a digital signal processing capability to allow not only the use of a plurality of the position indicators but also miniaturizes their size.
To further detail the signal processing means of the prior art, the received signal is sampled with a predetermined cycle, and converted to a digital signal to which the discrete Fourier transform operation is performed to calculate the amplitude and phase angle of a suitable frequency component included in the received signal. According to this, it can provide accurately calculated phase angle and amplitude of each of the frequencies for a single loop coil, which enables a plurality of position indicators having different resonance frequencies to be used simultaneously and the required mounting area reduced when compared with the means up to that time using ceramic filters.
However, in order to ensure the reliable discrete Fourier transform, a predetermined number of samples of the signal will be required during said predetermined period ( receiving period ). That is, there is a difficulty to sample using a quite high sampling frequency (clock). For example, in the disclosed embodiment of the prior art, 16 sinusoidal waves of 500 kHz are received during 32 .mu.sec of the receiving period to convert to 128 data of 6 bits. Thus a clock of 4 MHz is required to sample the signal for every 250 nsec. Practically a high speed analog to digital (A/D) converter is used. Both high speed and high resolution A/D converter, and a following arithmetic processing portion which conducts discrete Fourier transformation require a large circuit area and power dissipation in order to process such a high rate of data. Also, the high speed A/D converter has a relatively large number of many error factors, such as noise of its internal circuit as compared with a low speed A/D converter. Furthermore computer loads to control these processes are correspondingly increased so that an additional digital signal processor (DSP) is required to support the computer.
An alternative technique without using any high speed A/D converter can be used in which Fourier transform is performed on the receiving analog signal itself to obtain a signal having an amplitude and phase of a given frequency. The device for such technique is normally called a phase detector or synchronous detector, more specifically it has a capability to multiply the receiving signal with an alternating signal having a given frequency, and to integrate the resultant signals. Thereby an output value having corresponding amplitude and phase of the frequency component of the currently receiving signal can be obtained. The output will then be converted to digital data with which the coordinate values may be calculated.
However, as a common problem to analog phase detector, it may produce errors in its output signals due to the voltage offset, drifts, etc unique to the operational amplifier which is one of the components constituting the circuit, or gain variation between the components. When the amplitude and phase angles are calculated based on the values which include such errors, the following value such as coordinate value results to differ from the actual one, which in turn result in the accurate position not to be detected.